Exhaust-gas treatment apparatus and exhaust-gas treatment method

ABSTRACT

An object is to provide an exhaust-gas treatment apparatus capable of realizing a dissolved-salt spray method easily and at low cost. An exhaust-gas treatment apparatus that removes SO 2  and SO 3  contained in combustion exhaust gas includes a desulfurization apparatus based on the lime-gypsum method. Desulfurizing effluent, containing dissolved salt, from the desulfurization apparatus is sprayed to an upstream side of the desulfurization apparatus to remove SO 3 . A wet electrical dust precipitator may be provided downstream of the desulfurization apparatus. Furthermore, effluent from the wet electrical dust precipitator may be made to merge with the desulfurizing effluent from the desulfurization apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.12/593,300, filed Sep. 28, 2009, which is a National Phase filing ofPCT/JP2008/056300, filed Mar. 31, 2008, which is based on and claims thebenefit of priority in Japanese Patent Application No. 2007-092512,filed Mar. 20, 2007, the entire contents of all which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to an exhaust-gas treatment apparatus andan exhaust-gas treatment method for removing SO₃ from combustion exhaustgas.

BACKGROUND ART

Combustion exhaust gas including sulfur oxides, such as SO₂ and SO₃, isemitted from a combustion furnace in which fuels containing at least 0.5wt % sulfur content, such as heavy fuels or coal fuels, are combusted.

SO₃ is produced as a result of partial oxidation of SO₂ at hightemperatures. Therefore, the presence of SO₃ is about a few percentrelative to SO₂. Nevertheless, the concentration of exhaust SO₃ shouldbe controlled below a few ppm because SO₃ not only causes air heaters toclog or erode or chimney flues to erode but also causes blueish smoke tooccur when cooled and discharged from a chimney.

Whereas well-known methods for removing SO₃ include the ammoniainjection method in which ammonia gas is blown into combustion exhaustgas, the present inventors have proposed a method for sprayingdesulfurizing effluent including a dissolved salt, such as Na₂SO₄, intocombustion exhaust gas (hereinafter, referred to as the “dissolved-saltspray method”) as a technique for removing SO₃ easily and at a low cost(refer to Patent Document 1).

Patent Document 1:

Japanese Unexamined Patent Application, Publication No. 2006-326575

DISCLOSURE OF INVENTION

In the above-described Patent Document, however, the desulfurizationapparatus is intended mainly for use with the caustic soda method or thewater magnesite method. Desulfurization apparatuses based on the causticsoda method or the water magnesite method can process only about 200,000to 300,000 Nm³ of combustion exhaust gas, considering that the chemicalsolution to be used is expensive and that the effluent flow rate ishigh. Desulfurization apparatuses capable of processing a larger amountof combustion exhaust gas include those based on the lime-gypsum methodthat can process up to one million Nm³ or more. However, it is believedthat a dissolved salt in desulfurizing effluent is difficult to use withthe lime-gypsum method because this method involves poorly solublegypsum (CaSO₄) in the desulfurizing effluent, and hence, a dissolvedsalt has been obtained from a separately provided small-scaledesulfurization apparatus based on the caustic soda method (refer toFIG. 2 in Patent Document 1). Because such an approach isdisadvantageous in that additional cost is entailed in the new facility,there has been a growing need for an exhaust-gas treatment apparatusthat includes a desulfurization apparatus based on the lime-gypsummethod yet is still capable of realizing the dissolved-salt spray methodmore easily at lower cost.

Furthermore, it is desired to find an approach for realizing thedissolved-salt spray method without using dissolved salt indesulfurizing effluent or causing a significant increase in theoperating cost, such as that resulting from introduction of a newchemical solution.

The present invention has been conceived in light of thesecircumstances, and an object thereof is to provide an exhaust-gastreatment apparatus and an exhaust-gas treatment method capable ofrealizing the dissolved-salt spray method easily and at low cost.

Another object of the present invention is to provide an exhaust-gastreatment apparatus and an exhaust-gas treatment method that include adesulfurization apparatus based on the lime-gypsum method yet are stillcapable of realizing the dissolved-salt spray method without extensiveaddition of new facilities for supplying a dissolved salt.

Still another object of the present invention is to provide anexhaust-gas treatment apparatus and an exhaust-gas treatment method forrealizing the dissolved-salt spray method without causing a significantincrease in the operating cost, such as that resulting from introductionof a new chemical solution, even though dissolved salt is not used inthe desulfurizing effluent.

In order to solve the above-described problems, an exhaust-gas treatmentapparatus and an exhaust-gas treatment method according to the presentinvention provide the following solutions.

Specifically, an exhaust-gas treatment apparatus according to a firstaspect of the present invention is an exhaust-gas treatment apparatusthat removes SO₂ and SO₃ contained in combustion exhaust gas andincludes a desulfurization apparatus based on a lime-gypsum method and aspray section that sprays desulfurizing effluent from thedesulfurization apparatus to an upstream side of the desulfurizationapparatus.

SO₂ is mainly removed by the desulfurization apparatus based on thelime-gypsum method.

In the lime-gypsum method, lime is used as an absorbent. The presentinventors, as a result of intensive efforts, have focused attention onthe fact that Mg is contained in this lime. Consequently, the presentinventors have found that the advantage afforded by the use ofdesulfurizing effluent containing a dissolved salt whose main rawmaterial is this Mg is more than enough, even though the step ofseparating poorly soluble gypsum in the desulfurizing effluent isprovided.

The reason why SO₃ is removed by spraying desulfurizing effluent basedon the lime-gypsum method from the spray section is described below.

Desulfurizing effluent based on the lime-gypsum method contains MgSO₄,namely a dissolved salt, whose raw material is Mg in the form of lime.When sprayed, an aqueous solution containing this dissolved salt istransformed into atomized droplets, and the moisture content about thedissolved salt of these atomized droplets evaporates due to thecombustion exhaust gas. Because the moisture content of the atomizeddroplets is evaporated to produce dried particles of dissolved salt inthis manner, atomized particles of dissolved salt can be obtained. Then,as result of SO₃ coming into contact with the atomized, dried particlesof dissolved salt, contaminants are adsorbed and fixed, and then removedfrom the gas. In short, because a large number of small, dry particlesof dissolved salt can be produced by spraying an aqueous solutioncontaining dissolved salt, a sufficient surface area required to adsorbSO₃ contained in the combustion exhaust gas is ensured, therebypromoting the adsorption and fixation of contaminants.

Furthermore, because desulfurizing effluent is sprayed to an upstreamside of the desulfurization apparatus, the dissolved salt in the sprayeddesulfurizing effluent adsorbs SO₃ in the combustion exhaust gas, iscollected in the desulfurization apparatus, is dissolved again, and isrecovered as dissolved salt. At this time, as a result of SO₃ reactingwith limestone, gypsum (CaSO₄.2H₂O) is produced. Then, the recovereddissolved salt is sprayed again. As described above, because thedissolved salt in desulfurizing effluent can be circulated, theconcentration of dissolved salt in the desulfurizing effluent can beincreased. In addition, because it is not necessary to introduce a newchemical, the operating costs can be reduced considerably.

Because the moisture content needs to evaporate while sprayed dropletsof desulfurizing effluent are floating, an environment where the exittemperature of the spray section is equal to or higher than the waterevaporation temperature is preferable. Furthermore, the SO₃ dew-pointtemperature or higher at which a reaction with the dissolved salt occursis desirable. This is because at temperatures below the dew point, theSO₃ gas is transformed into SO₃ mist and adsorption to fine particles ofdry dissolved salt does not occur easily, thereby decreasing the removalcapability. Therefore, the exit temperature of the spray section shouldbe 130° C. or more, preferably 140° C. or more.

Furthermore, a two-fluid nozzle is preferable as the spray sectionbecause droplets to be sprayed can be atomized. It is preferable thatdroplets of desulfurizing effluent to be sprayed have diameters suchthat the moisture content evaporates while sprayed droplets of anaqueous solution are floating; for example, the diameter is preferably10 μm to 100 μm, more preferably about 20 to 50 μm.

Furthermore, in the exhaust-gas treatment apparatus according to thefirst aspect of the present invention, a wet electrical dustprecipitator may be provided at a downstream side of the desulfurizationapparatus.

Dust and SO₃ that have not been captured by the preceding dryelectrostatic precipitator, dissolved-salt spray apparatus, ordesulfurization apparatus based on the lime-gypsum method can be removedby this wet electrical dust precipitator. The facility capacity(required electrode area) of the wet electrical dust precipitator isdetermined by the SO₃ concentration at the inlet of the wet electricaldust precipitator. Therefore, in combination with the dissolved-saltspray apparatus, the capacity of the wet electrical dust precipitatorcan be decreased.

Furthermore, in the exhaust-gas treatment apparatus with theabove-described structure, effluent from the wet electrical dustprecipitator may be made to merge with the desulfurizing effluent fromthe desulfurization apparatus.

Alkaline solutions such as a NaOH aqueous solution and a Mg (OH)₂aqueous solution are supplied for neutralization to the wet electricaldust precipitator. After neutralization, these alkaline solutions aretransformed into aqueous solutions containing dissolved salts such asNa₂SO₄ and MgSO₄ in the form of effluent. The concentration of dissolvedsalt can be increased by allowing this post-neutralization effluent tomerge with the desulfurizing effluent. The SO₃ removal efficiency can befurther enhanced by spraying desulfurizing effluent containing suchpost-neutralization effluent to an upstream side of the desulfurizationapparatus in this manner.

Furthermore, because a neutralizer supplied to the wet electrical dustprecipitator is used, it is not necessary to deliver an additional newchemical. This prevents the operating costs from rising.

In the present invention, the wet electrical dust precipitator includesa dielectric gas cleaning apparatus.

Furthermore, in either of the exhaust-gas treatment apparatusesdescribed above, a dry electrostatic precipitator may be provided at anupstream side of the spray section, and a residue of desulfurizingeffluent excluding desulfurizing effluent introduced to the spraysection may be sprayed to an upstream side of the dry electrostaticprecipitator, wherein the residue and the desulfurizing effluentintroduced to the spray section constitute the desulfurizing effluentfrom the desulfurization apparatus.

The residue of desulfurizing effluent excluding the desulfurizingeffluent introduced to the spray section is sprayed to an upstream sideof the dry electrostatic precipitator. The moisture content of thedesulfurizing effluent that has been sprayed to an upstream side of thedry electrostatic precipitator is evaporated by the sensible heat of thecombustion exhaust gas flowing through the chimney flue, leaving drysolid content as a residue. This dry solid content is then removed bythe dry electrostatic precipitator. Therefore, it is not necessary toseparately process the desulfurizing effluent from the desulfurizationapparatus. That is, a non-effluent treatment of the desulfurizingeffluent is realized.

Furthermore, in either of the exhaust-gas treatment apparatusesdescribed above, a heat exchanger that recovers heat from a combustionexhaust gas upstream of the desulfurization apparatus may be provided.

Heat can be used effectively by recovering the heat of the combustionexhaust gas with the heat exchanger. For example, the heat recovered bythe heat exchanger can be supplied to boiler feedwater.

Furthermore, in the exhaust-gas treatment apparatus with theabove-described structure, the heat recovered by the heat exchanger maybe given to combustion exhaust gas before being released to theatmosphere.

White smoke can be prevented from occurring by increasing, by means ofthe heat exchanger, the temperature of the combustion exhaust gas beforebeing released to the atmosphere.

Furthermore, in either of the exhaust-gas treatment apparatusesdescribed above, a sedimentation tank that separates solid content ofthe desulfurizing effluent from the desulfurization apparatus may beprovided, and a separated liquid isolated by the sedimentation tank maybe supplied to the spray section.

Because of the desulfurization apparatus based on the lime-gypsummethod, desulfurizing effluent contains gypsum in the form of solidcontent (CaSO₄). This solid content is separated by the sedimentationtank to obtain a separated liquid from which solid content has beenremoved. Then, because this separated liquid is sprayed, the risk ofclogging the feed opening of the spray section with solid content can bereduced.

Furthermore, in the exhaust-gas treatment apparatus with theabove-described structure, a membrane separation apparatus thatmembrane-separates the separated liquid from the sedimentation tank maybe provided.

The solid content can be removed at a high level by supplying theseparated liquid to the membrane separation apparatus, thereby furtherreducing the risk of clogging the feed opening of the spray section.

Furthermore, in either of the exhaust-gas treatment apparatusesdescribed above, a salt-level measuring section that measures aconcentration of dissolved salt in desulfurizing effluent supplied tothe spray section may be provided.

Dissolved salt can be sprayed at an appropriate salt level by measuringthe concentration of dissolved salt in the desulfurizing effluentsupplied to the spray section and, for example, by increasing/decreasingthe volume of effluent.

A densimeter or a conductivity meter can be used for the salt-levelmeasuring section.

Furthermore, in either of the exhaust-gas treatment apparatusesdescribed above, a dissolved-salt-level adjusting section may beprovided in a flow channel of desulfurizing effluent supplied to thespray section.

As a result of the dissolved-salt-level adjusting section beingprovided, desulfurizing effluent to be sprayed can be adjusted toexhibit a desired concentration of dissolved salt even when the desiredconcentration of dissolved salt cannot be attained, such as when theoperation is started.

The dissolved-salt-level adjusting section includes those supplyingdissolved-salt producing materials serving as raw materials forproducing dissolved salts such as NaOH and Mg (OH)₂, as well asdissolved salts such as Na₂SO₄ and MgSO₄.

Furthermore, an exhaust-gas treatment apparatus according to a secondaspect of the present invention is an exhaust-gas treatment apparatusthat removes SO₃ and dust contained in a combustion exhaust gas andincludes a dry electrostatic precipitator that removes dust; adust-dissolving section that supplies an alkaline solution to dustcollected by the dry electrostatic precipitator to dissolve the dust;and a spray section that sprays effluent from the dust-dissolvingsection to an upstream side of the dry electrostatic precipitator.

Dust is removed by the dry electrostatic precipitator. Furthermore, SO₃is removed by the dust solution (effluent from the dust-dissolvingsection) sprayed from the spray section. The reason why SO₃ is removedby a dust solution is described below.

Because an alkaline solution for neutralization is supplied to thedust-dissolving section, the dust solution from the dust-dissolvingsection contains dissolved salt, as a solute, originating from thealkaline solution such as NaOH or Mg(OH)₂. When sprayed, an aqueoussolution containing this dissolved salt is transformed into atomizeddroplets, and the moisture content about the dissolved salt of theseatomized droplets evaporates due to the combustion exhaust gas. Becausethe moisture content of the atomized droplets is evaporated to producedried particles of dissolved salt in this manner, atomized particles ofdissolved salt can be obtained. Then, as result of SO₃ coming intocontact with the atomized, dried particles of dissolved salt,contaminants are adsorbed and fixed, and removed from the gas. In short,because a large number of small, dry particles of dissolved salt can beproduced by spraying an aqueous solution containing dissolved salt, asufficient surface area required to adsorb SO₃ contained in thecombustion exhaust gas is ensured, thereby promoting the adsorption andfixation of contaminants.

In addition, because effluent is sprayed at an upstream side of the dryelectrostatic precipitator, the dissolved salt in the effluent is driedinto solid dissolved salt by the sensible heat of the combustion exhaustgas. This solid dissolved salt adsorbs SO₃ and is captured by the dryelectrostatic precipitator. The solid dissolved salt captured by the dryelectrostatic precipitator is discharged together with dust via an ashtreatment facility. By introducing some of the solid dissolved salt tothe dust-dissolving section and supplying an alkaline solution, anaqueous solution containing dissolved salt can be produced again andthen sprayed again to an upstream side of the dry electrostaticprecipitator. Because dissolved salt is circulated in this manner, theconcentration of dissolved salt in the effluent can be increased.Furthermore, as a result of dissolved salt being circulated, it is notnecessary to deliver many alkaline solutions for SO₃ removal. This helpsreduce the amount of chemicals used.

Because the moisture content needs to evaporate while sprayed dropletsof effluent are floating, an environment where the exit temperature ofthe spray section is equal to or higher than the water evaporationtemperature is preferable. Furthermore, the SO₃ dew-point temperature orhigher at which a reaction with the dissolved salt occurs is desirable.This is because at temperatures below the dew point, the SO₃ gas istransformed into SO₃ mist and adsorption to fine particles of drydissolved salt does not occur easily, thereby decreasing the removalcapability. Therefore, the exit temperature of the spray section shouldbe 130° C. or more, preferably 140° C. or more.

Furthermore, a two-fluid nozzle is preferable as the spray section inthat droplets to be sprayed can be atomized. It is preferable thatdroplets of effluent to be sprayed have diameters such that the moisturecontent evaporates while sprayed droplets of an aqueous solution arefloating; for example, the diameter is preferably 10 μm to 100 μm, morepreferably about 20 to 50 μm.

In addition, any concentration of dissolved salt can be obtained byadjusting the concentration of an alkaline solution supplied as aneutralizer.

Furthermore, in the exhaust-gas treatment apparatus according to thesecond aspect of the present invention, a desulfurization apparatusbased on a lime-gypsum method may be provided downstream of the dryelectrostatic precipitator, and desulfurizing effluent from thedesulfurization apparatus may be introduced to the dust-dissolvingsection.

Desulfurizing effluent can be used for dust dissolution by introducingthe desulfurizing effluent to the dust-dissolving section.

Furthermore, if all desulfurizing effluent is used for dust dissolution,a non-effluent treatment of desulfurizing effluent can be realized.

In addition, because the desulfurizing effluent contains dissolved salt,the concentration of dissolved salt in the effluent sprayed to thecombustion exhaust gas can be increased.

Furthermore, in the exhaust-gas treatment apparatus with theabove-described structure, a gypsum separator that separates gypsum fromthe desulfurizing effluent from the desulfurization apparatus may beprovided, and the effluent discharged from the dust-dissolving sectionmay be supplied to the gypsum separator.

A dust solution is supplied to the gypsum separator. Because the solidcontent is separated in the gypsum separator, gypsum mixed withinsoluble matter in the dust solution is produced. Because reuse ordiscarding is performed in the form of gypsum mixed with dischargeddust, the need for separately providing a dust treatment apparatus iseliminated, thus simplifying the facility.

In addition, an exhaust-gas treatment method according to a third aspectof the present invention is an exhaust-gas treatment method of removingSO₂ and SO₃ contained in combustion exhaust gas and includes sprayingdesulfurizing effluent from a desulfurization apparatus based on alime-gypsum method to an upstream side of the desulfurization apparatus.

Furthermore, an exhaust-gas treatment method according to a fourthaspect of the present invention includes supplying an alkaline solutionto dust collected by a dry electrostatic precipitator for removing dustto dissolve the dust; and spraying effluent resulting after the dust isdissolved to an upstream side of the dry electrostatic precipitator.

More specifically, an alkaline solution is supplied to some of the dustcollected by the dry electrostatic precipitator for removing dust. Thealkaline solution not only neutralizes the liquid pH but also allows theconcentration of salt to be adjusted.

Because dissolved salt whose raw material is Mg present in thedesulfurizing effluent from the desulfurization apparatus based on thelime-gypsum method is sprayed into the combustion exhaust gas to removeSO₃, an exhaust-gas treatment apparatus can be constructed easily at lowcost without having to separately provide an apparatus for producingdissolved salt.

In addition, because the dissolved salt sprayed into the combustionexhaust gas circulates together with desulfurizing effluent, it is notnecessary to provide additional chemical solutions. This considerablyreduces the operating costs.

Furthermore, because effluent, from the dust-dissolving section,containing dissolved salt whose raw material is an alkaline solution issprayed to an upstream side of the dry electrostatic precipitator toremove SO₃, an exhaust-gas treatment apparatus can be constructed easilyand at low cost without having to separately provide an apparatus forproducing dissolved salt.

In addition, because dissolved salt is recirculated for use bycollecting dissolved salt with the electrical dust precipitator, it isnot necessary to deliver many alkaline solutions for SO₃ removal. Thiscan reduce the operating costs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram depicting parts surrounding adesulfurization apparatus and a wet electrical dust precipitator in FIG.1.

FIG. 3 is a schematic diagram depicting a structure having an additionalmembrane separation apparatus downstream of a sedimentation tank in FIG.2.

FIG. 4 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a second embodiment of the present invention.

FIG. 5 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a third embodiment of the present invention.

FIG. 6 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a fourth embodiment of the present invention.

FIG. 7 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a fifth embodiment of the present invention.

FIG. 8 is a schematic diagram depicting an exhaust-gas treatmentapparatus according to a sixth embodiment of the present invention.

EXPLANATION OF REFERENCE SIGNS

-   11: dry electrostatic precipitator-   13: desulfurization apparatus-   15: wet electrical dust precipitator-   31: sedimentation tank-   35: two-fluid nozzle (spray section)-   54: membrane separation apparatus

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments according to the present invention will now be describedwith reference to the drawings.

First Embodiment

A first embodiment according to the present invention will be describedbelow.

FIG. 1 shows an exhaust-gas treatment apparatus according to thisembodiment.

The exhaust-gas treatment apparatus is disposed in a chimney fluedownstream of a boiler 3 provided with a combustion furnace and includesa denitration apparatus 7; an air heater 9; a dry electrostaticprecipitator (D-EP) 11; a desulfurization apparatus 13 based on thelime-gypsum method; and a wet electrical dust precipitator (W-EP) 15.

The boiler 3 is, for example, an oil-fired boiler for combusting heavyoil containing at least 0.5 wt % sulfur content.

The denitration apparatus 7 removes nitrogen oxides (NOx) contained inthe combustion exhaust gas from the boiler 3.

The air heater 9 performs heat exchange between combustion air suppliedto the boiler 3 and combustion exhaust gas. Because of this, thecombustion air is heated by the sensible heat of the combustion exhaustgas and supplied to the boiler 3.

The dry electrostatic precipitator 11 electrically captures dust in thecombustion exhaust gas. The dust collected by the dry electrostaticprecipitator 11 is processed by an ash treatment apparatus (dusttreatment apparatus) 17.

The desulfurization apparatus 13 mainly removes SO₂ in the combustionexhaust gas by the lime-gypsum method. The desulfurizing effluent fromthe desulfurization apparatus 13 is introduced to a gypsum separator 29,where gypsum 16 is separated. The desulfurizing effluent flowing out ofthe gypsum separator 29 is sprayed upstream of the desulfurizationapparatus 13 as desulfurizing effluent containing dissolved salt 19.More specifically, some of the desulfurizing effluent is sprayed by atwo-fluid nozzle (refer to reference numeral 35 in FIG. 2) serving as aspray section disposed at a position close to the desulfurizationapparatus 13 in the chimney flue between the dry electrostaticprecipitator 11 and the desulfurization apparatus 13. The two-fluidnozzle changes liquid into very fine particles by means of pressurizedair, and the sprayed desulfurizing effluent is preferably 10 μm to 100μm, more preferably 20 to 50 μm, and still more preferably 25 to 35 μmin diameter.

Because the moisture content needs to evaporate while droplets ofsprayed desulfurizing effluent are floating, the environment ispreferably at the water evaporating temperature or higher. Furthermore,the temperature is preferably equal to or higher than the SO₃ dew-pointtemperature at which a reaction with dissolved salt occurs. This isbecause, at a temperature below the dew point, the SO₃ gas istransformed into SO₃ mist and it becomes difficult to adhere to veryfine particles of dry dissolved salt, thus decreasing the removalcapability. For this reason, the exit temperature of the two-fluidnozzle is 130° C. or higher, and preferably 140° C. or higher.

A residue of desulfurizing effluent that has not been sprayed upstreamof the desulfurization apparatus 13 is processed by an effluenttreatment apparatus 20 and then discharged outside.

The wet electrical dust precipitator 15 removes dust and SO₃. An NaOHaqueous solution for neutralization is supplied to the wet electricaldust precipitator 15, as described below, and neutralized effluentmerges with the desulfurizing effluent from the desulfurizationapparatus 13.

A dielectric gas cleaning apparatus that dielectrically polarizesdielectric particles, such as sprayed water, to capture precharged dustor SO₃ by the Coulomb force acting among the dielectric particles may beused as the wet electrical dust precipitator 15.

FIG. 2 shows the structure downstream of the dry electrostaticprecipitator 11 shown in FIG. 1. More specifically, the figure depictsthe desulfurization apparatus 13 and the wet electrical dustprecipitator 15.

The desulfurization apparatus 13 includes an absorbent spray 21 thatsprays an absorbent containing CaCO₃, a reaction layer 22 disposed belowthe absorbent spray 21, and a reservoir 24 disposed below the reactionlayer 22.

A liquid-jet column, a spray tower, or a plastic packed bed may be usedfor the reaction layer 22.

An absorbent feed pump 26 is disposed between the absorbent spray 21 andthe reservoir 24, and the absorbent in the reservoir 24 is sucked bythis absorbent feed pump 26 into the absorbent spray 21 and circulated.

Limestone is supplied from a limestone feed section 25 to the reservoir24, so that absorbent slurry containing CaCO₃ is produced. Becauselimestone contains about 0.5 to 2 wt % Mg, Mg also dissolves as a resultof the limestone being dissolved in the desulfurization apparatus andreacts with SO₂ in the gas, thereby producing a Mg dissolved salt.Desulfurizing effluent containing the produced MgSO₄ is introduced viaan eductor pump 27 to the gypsum separator 29, together with gypsum.

CaSO₄ is also dissolved in some of the desulfurizing effluent (thesolubility of CaSO₄ is 2000 mg/l).

The gypsum in the form of solid content is separated from liquid in thegypsum separator 29. The separated gypsum is discharged outside forreuse. The liquid separated by the gypsum separator 29 is introduced asdesulfurizing effluent into a sedimentation tank 31. Particulate solidcontent that has not been separated in the gypsum separator 29 isfurther separated in the sedimentation tank 31. In the sedimentationtank 31, particulate solid content with high specific gravity issedimented. The sedimented liquid containing the sedimented particulatesolid content is taken out from below and is returned to the reservoir24.

The supernatant liquor of the desulfurizing effluent that has beenseparated in the sedimentation tank 31 is taken out by a dissolved-saltspray pump 33 and is introduced to a two-fluid nozzle 35. At this time,the rest of desulfurizing effluent is branched off, is processed by aneffluent treatment apparatus 37, and is then discharged outside.

Desulfurizing effluent (dissolved salt) is sprayed from the two-fluidnozzle 35 to a dissolved-salt spray area 36 in the chimney flueextending substantially vertically.

An antechamber 31 a of the sedimentation tank 31 is provided with a saltlevel sensor (salt-level measuring section) 50 so that the concentrationof dissolved salt in the desulfurizing effluent to be supplied to thetwo-fluid nozzle can be measured. For example, a densimeter or aconductivity meter may be used for the salt level sensor 50. Theantechamber 31 a of the sedimentation tank 31 is also provided with adissolved-salt feed section (dissolved-salt-level adjusting section) 52.Dissolved-salt producing materials serving as raw materials forproducing dissolved salts, such as NaOH and Mg(OH)₂, and dissolved saltssuch as Na₂SO₄ and MgSO₄ are supplied from the dissolved-salt feedsection 52.

A washing-water tank 40 is disposed near the wet electrical dustprecipitator 15. NaOH is supplied to this washing-water tank 40 from analkaline-water feed section 42 in order to prevent a pH decreaseresulting from SO₂ and SO₃ being adsorbed and removed in the wetelectrical dust precipitator 15. The washing water, containing NaOH,pooled in the washing-water tank 40 is introduced by a washing-waterfeed pump 44 to the wet electrical dust precipitator 15. The corrosionof components in the apparatus is prevented by supplying this washingwater for neutralization to the wet electrical dust precipitator 15. Inthe wet electrical dust precipitator 15, a dissolved salt (Na₂SO₄) isgenerated as a result of NaOH in the washing water reacting with SO₂ andSO₃ in the combustion exhaust gas. In this manner, the washing waterthat has passed through the wet electrical dust precipitator 15 afterneutralizaion is discharged to the washing tank 40. Therefore, Na₂SO₄ inthe form of dissolved salt is present in the effluent discharged to thewashing-water tank 40. While the effluent pooled in the washing-watertank 40 is re-introduced by the washing-water feed pump 44 to the wetelectrical dust precipitator 15, some of the effluent is supplied to thereservoir 24 of the desulfurization apparatus 13. By allowing some ofthe effluent from the wet electrical dust precipitator 15 to merge withthe desulfurizing effluent from the desulfurization apparatus 13 in thismanner, Na₂SO₄ in the form of dissolved salt present in the effluentfrom the wet electrical dust precipitator 15 is added to thedesulfurizing effluent. As a result, the concentration of dissolved saltin the desulfurizing effluent increases.

The exhaust-gas treatment apparatus with the above-described structureis operated as follows. First, the operation during steady operation isdescribed, followed by the operation during initial operation.

<During Steady Operation>

When heavy oil containing a large sulfur content is combusted in theboiler 3, combustion exhaust gas containing SO₂ is discharged to thedownstream chimney flue. Some (a few percent) of the SO₂ is oxidizedinto SO₃ at high-temperature sections in the boiler 3 and thedenitration apparatus 7.

The combustion exhaust gas that has passed through the denitrationapparatus 7 gives up some of its sensible heat to combustion air throughthe air heater 9. At this time, the temperature of combustion exhaustgas decreases to about 160 to 200° C.

The combustion exhaust gas that has passed through the air heater 9 isintroduced to the dry electrostatic precipitator 11, and dust in thecombustion exhaust gas is removed. The removed dust is processed by theash treatment apparatus 17 and is discharged outside.

The combustion exhaust gas that has passed through the dry electrostaticprecipitator 11 is sprayed with desulfurizing effluent (dissolved salt)from the two-fluid nozzle 35 in the dissolved-salt spray area 36 (referto FIG. 2) when flowing through the chimney flue before thedesulfurization apparatus 13. Most of the SO₃ is removed bydesulfurizing effluent atomized by the two-fluid nozzle 35.Specifically, when sprayed with desulfurizing effluent containing aMgSO₄ aqueous solution and a Na₂SO₄ aqueous solution in the form ofdissolved salt, SO₃ is removed from the gas as a result of beingadsorbed to and fixed with the atomized dry particles of dissolved salt.MgSO₄ in the desulfurizing effluent is based on the limestone suppliedas a raw material to the desulfurization apparatus 13, whereas Na₂SO₄ inthe desulfurizing effluent is based on NaOH added as a raw material towashing water that is supplied to the wet electrical dust precipitator15.

Reaction formulae in the process of removing SO₃ with particles ofdissolved salts are shown below as one example. Because these reactionformulae vary depending on the amount of sprayed dissolved salts, theactual SO₃-removal mechanism is not limited to these reaction formulae.Particles of dissolved salts (MgSO₄ and Na₂SO₄) that have been dried asa result of being sprayed into the combustion gas are subjected to thereaction processes with SO₃, specified in the formulae below, therebyproducing (Mg)_(1/2)HSO₄.H₂O(solid) and NaHSO₄.H₂O(solid).

MgSO₄(dissolved)+SO₃+3H₂O→2(Mg)_(1/2)HSO₄.H₂O(solid)

Na₂SO₄(dissolved)+SO₃+3H₂O→2NaHSO₄.H₂O(solid)

Combustion exhaust gas from which most SO₃ has been removed withdissolved salts supplied from the two-fluid nozzle 35 is introduced tothe desulfurization apparatus 13.

In the desulfurization apparatus 13, SO₂ is removed with an absorbentthat is sprayed by the absorbent spray 21. The solid dissolved salts((Mg)_(1/2)SO4.H₂O and NaHSO₄.H₂O) that have been sprayed from thetwo-fluid nozzle 35 to adsorb SO₃ are also introduced to the reservoir24 together with the absorbent. In the reservoir 24, solid dissolvedsalts are reproduced via the following reaction processes.

2(Mg)₁.2HSO₄.H₂O(solid)+CaCO₃

→MgSO₄(dissolved)+CaSO₄.2H₂O+CO₂+H₂O

2NaHSO₄.H₂O(solid)+CaCO₃

→Na₂SO₄ (dissolved)+CaSO₄.2H₂O+CO₂+H₂O

The reproduced dissolved salts (MgSO₄ and Na₂SO₄) are introduced to thetwo-fluid nozzle 35 again and used to remove SO₃.

Because the dissolved salts in the desulfurizing effluent are made tocirculate from the reservoir 24 to the gypsum separator 29, thesedimentation tank 31, the two-fluid nozzle 35, and the reservoir 24 inthat order as described above, the dissolved salts in the desulfurizingeffluent are conserved, thereby eliminating the need for extra operatingcosts. In addition, the concentration of dissolved salt can be increasedby adjusting limestone supplied from the limestone feed section 25 andthe flow rate of effluent discharged to the effluent treatment facility37.

Desulfurizing effluent (dissolved salt) supplied from the two-fluidnozzle 35 has a concentration appropriate for adsorbing SO₃ according tovarious conditions. For this reason, the concentration of dissolved saltin the desulfurizing effluent supplied from the two-fluid nozzle 35 iscontrolled while the concentration of dissolved salt is being monitoredwith the salt level sensor 50. More specifically, the flow rate ofdesulfurizing effluent that is sent to the effluent treatment apparatus37 for disposal is adjusted. For example, if the concentration ofdissolved salt is low, the rate of flow into the effluent treatmentapparatus is decreased to increase the concentration of dissolved salt.In contrast, if the concentration of dissolved salt is high, the rate offlow into the effluent treatment apparatus is increased and salt in thecirculating desulfurizing effluent is discarded to decrease theconcentration of dissolved salt.

The concentration of dissolved salt in the desulfurizing effluent ispreferably controlled to range from 30000 mg/l to 100000 mg/l.

Combustion exhaust gas from which SO₂ has been removed in thedesulfurization apparatus 13 is introduced into the wet electrical dustprecipitator 15, where remaining dust, SO₃, and so forth are removedfrom the combustion exhaust gas, which is then discharged outside via achimney (not shown in the figure).

<During Initial Operation>

Unlike the above-described case, the desired concentration of dissolvedsalt may not be obtained during initial operation because ofinsufficient circulation of dissolved salt. If this is the case, thedesired concentration of dissolved salt is achieved from the early stageby supplying dissolved-salt producing materials and dissolved salt fromthe dissolved-salt feed section 52. Once the concentration of dissolvedsalt is increased, it is not necessary to further supply dissolved-saltproducing materials or dissolved salt because dissolved salt iscirculated and conserved as described above.

With this embodiment, the following advantages can be afforded.

Because a dissolved salt whose raw material is Mg present in thedesulfurizing effluent from the desulfurization apparatus 13 based onthe lime-gypsum method is sprayed into combustion exhaust gas to removeSO₃, the exhaust-gas treatment apparatus can be constructed easilywithout having to provide a separate apparatus for producing dissolvedsalt.

Furthermore, because dissolved salt that is sprayed into combustionexhaust gas is dissolved again in the desulfurization apparatus afterbeing used to remove SO₃, it is not necessary to supply another chemicalsolution to produce dissolved salt. This considerably reduces theoperating cost.

In addition, by allowing effluent from the wet electrical dustprecipitator 15 to merge with the desulfurizing effluent, a dissolvedsalt whose raw material is Na present in the effluent can be used toremove SO₃. Because of this, the concentration of dissolved salt can beincreased easily, thereby achieving the desired concentration ofdissolved salt. Furthermore, because Na present in the effluent from thewet electrical dust precipitator 15 is produced from NaOH, serving as araw material, used for neutralization, the need for a new chemicalsolution is eliminated. This prevents the operating cost from rising.

In addition, because gypsum in the form of solid content (CaSO₄) presentin the desulfurizing effluent is separated by the sedimentation tank 31so that separated liquid from which solid content has been removed issprayed, the risk of clogging a feed opening of the two-fluid nozzle 35with solid content can be reduced.

As shown in FIG. 3, a membrane separation apparatus 54 may be provideddownstream of the sedimentation tank 31 to further remove solid contentin the desulfurizing effluent. In the figure, reference symbol 56denotes a tank that is disposed downstream of the membrane separationapparatus 54 to pool membrane-separated desulfurizing effluent(dissolved salt), reference symbol 33 a denotes a pump for supplyingsupernatant liquor in the sedimentation tank 31 to the membraneseparation apparatus, and reference symbol 33 b denotes a pump forsupplying desulfurizing effluent pooled in the tank 56 to the two-fluidnozzle 35.

Because solid content is further removed from the desulfurizing effluentby the membrane separation apparatus 54 as described above, the risk ofclogging the feed opening of the two-fluid nozzle 35 with solid contentcan be further reduced.

Second Embodiment

A second embodiment according to the present invention will be describedwith reference to FIG. 4. This embodiment differs from the firstembodiment in that desulfurizing effluent is subjected to a non-effluenttreatment. The other components are the same as those in the firstembodiment. Therefore, the same components as those in the firstembodiment are denoted with the same reference numerals, and hence, adescription thereof will be omitted.

As shown in FIG. 4, a residue of desulfurizing effluent, excluding thedesulfurizing effluent that is supplied from the desulfurizationapparatus 13 and introduced into the two-fluid nozzle 35 (refer to FIG.2), is sprayed upstream of the dry electrostatic precipitator 11 via aflow channel 60. The moisture content of the desulfurizing effluent thathas been sprayed upstream of the dry electrostatic precipitator 11 isevaporated by sensible heat of the combustion exhaust gas flowingthrough the chimney flue, and consequently, dry and solid content isproduced as a residue. This dry, solid content is then removed by thedry electrostatic precipitator 11. Therefore, it is no longer necessaryto process the desulfurizing effluent from the desulfurization apparatus13 by the effluent treatment apparatus 20 (refer to FIG. 1); in short, anon-effluent treatment of desulfurizing effluent can be realized.

Third Embodiment

A third embodiment according to the present invention will be describedwith reference to FIG. 5. This embodiment differs from the firstembodiment in that a heat exchanger for heat exchange of combustionexhaust gas is added. The other components are the same as those in thefirst embodiment. Therefore, the same components as those in the firstembodiment are denoted with the same reference numerals, and hence, adescription thereof will be omitted. This embodiment is also applicableto the second embodiment.

As shown in FIG. 5, a heat exchanger (gas-gas heater (GGH)) 62 thatperforms heat exchange between the combustion exhaust gas from the dryelectrostatic precipitator 11 to the desulfurization apparatus 13 andthe combustion exhaust gas before being released to the atmospheredownstream of the wet electrical dust precipitator 15 is provided.Although the heat exchanger 62 is indicated at two locations in thefigure, it does not mean that two separate heat exchangers 62 areprovided; it just means that heat exchange is performed between theseheat exchangers 62. White smoke can be prevented from occurring byincreasing, with the heat exchanger 62 described above, the temperatureof combustion exhaust gas before it is released to the atmosphere.

If it is not necessary to perform heat exchange with the combustionexhaust gas before releasing it to the atmosphere, heat may be extractedby the heat exchanger from the combustion exhaust gas upstream of thedesulfurization apparatus 13 so that the extracted heat can be suppliedto, for example, boiler feedwater for effective utilization of the heat.

Fourth Embodiment

A fourth embodiment according to the present invention will be describedwith reference to FIG. 6. This embodiment differs from the firstembodiment in that, unlike the first embodiment, SO₃ removal is notperformed by means of desulfurizing effluent. The boiler 3, thedenitration apparatus 7, the air heater 9, the dry electrostaticprecipitator 11, the desulfurization apparatus 13, the wet electricaldust precipitator 15, and the ash treatment apparatus 17 are the same astheir counterparts in the first embodiment, and hence descriptionsthereof will be omitted.

Some of the dust captured by the dry electrostatic precipitator 11 isintroduced to an ash-dissolving section (dust-dissolving section) 70. Analkaline aqueous solution is supplied to the ash-dissolving section 70from an alkaline-aqueous-solution feed section 72 for neutralizing ash.For an alkaline aqueous solution, an NaOH aqueous solution, Mg(OH)₂, andso forth can be used. Effluent resulting after ash has been dissolvedand neutralized in the ash-dissolving section 70 is sprayed as dissolvedsalt 74 towards the upstream end of the dry electrostatic precipitator11, namely, the combustion exhaust gas in the chimney flue between theair heater 9 and the dry electrostatic precipitator 11. The feature thatthe two-fluid nozzle 35 (refer to FIG. 2) is used when the dissolvedsalt 74 is to be sprayed is the same as in the first embodiment.

The reaction process of removing SO₃ in the combustion exhaust gas isthe same as in the first embodiment because the dissolved salts ineffluent use Na and Mg as their raw materials in the first embodiment.More specifically, when effluent containing dissolved salt is sprayed,the effluent is transformed into atomized droplets, and then themoisture content about the dissolved salt of these atomized droplets isevaporated with combustion exhaust gas. Because dried particles ofdissolved salt are produced by evaporating the moisture content of theatomized droplets as described above, atomized particles of dissolvedsalt can be obtained. Then, as a result of SO₃ coming into contact withthe atomized and dried particles of dissolved salt, contaminants areadsorbed and fixed, and thus removed from the gas. In short, when anaqueous solution containing dissolved salt is sprayed, many small driedparticles of dissolved salt can be produced. Therefore, a sufficientsurface area required to adsorb SO₃ contained in the combustion exhaustgas is ensured, thereby promoting the adsorption and fixation ofcontaminants.

With this embodiment, the following advantages are afforded.

Because effluent is sprayed upstream of the dry electrostaticprecipitator 11, the dissolved salt in the effluent is dried by sensibleheat of the combustion exhaust gas into solid dissolved salt, whichadsorbs SO₃ and is captured by the dry electrostatic precipitator 11.The solid dissolved salt captured by the dry electrostatic precipitatoris introduced to the ash-dissolving section 70 together with dust, isprocessed, and is sprayed again. Because dissolved salt is circulated inthis manner, the concentration of dissolved salt in the effluent can beincreased. This means that a concentration of dissolved salt appropriatefor SO₃ removal can be achieved.

In addition, because effluent is circulated, it is not necessary todeliver many alkaline solutions for the purpose of SO₃ removal. This canreduce the amount of chemicals used.

Furthermore, the concentration of dissolved salt in the effluent to besprayed into combustion exhaust gas can be adjusted easily by adjustingthe concentration of an alkaline aqueous solution supplied from thealkaline-aqueous-solution feed section 72.

Although a desulfurization apparatus based on the lime-gypsum method isused in this embodiment, the present invention is not limited to this. Adesulfurization apparatus based on the caustic soda method or the watermagnesite method is also possible.

Fifth Embodiment

A fifth embodiment according to the present invention will be describedwith reference to FIG. 7. This embodiment differs from the fourthembodiment in that desulfurizing effluent from the desulfurizationapparatus 13 is delivered to the ash-dissolving section 70. Because theother components are the same, the same components are denoted with thesame reference numerals, and hence a description thereof will beomitted.

As shown in FIG. 7, all of the desulfurizing effluent from thedesulfurization apparatus 13 is supplied to the ash-dissolving section70 via a flow channel 75. By doing so, not only can the desulfurizingeffluent be used to dissolve ash, but also a non-effluent treatment ofthe desulfurizing effluent can be realized.

In addition, because the desulfurizing effluent contains dissolved salt,the concentration of dissolved salt in the effluent to be sprayed intocombustion exhaust gas can be increased.

Sixth Embodiment

A sixth embodiment according to the present invention will be describedwith reference to FIG. 8. This embodiment differs from the fifthembodiment in that the ash treatment apparatus 17 is omitted. Becausethe other components are the same, the same components are denoted withthe same reference numerals, and hence a description thereof will beomitted.

As shown in FIG. 8, the ash treatment apparatus 17 (refer to FIG. 7) isnot provided in this embodiment. Dust captured by the dry electrostaticprecipitator 11 is supplied from the ash-dissolving section 70 via aflow channel 77 to the gypsum separator 29, together with gypsum slurryfrom the desulfurization apparatus 13. By doing so, dust is processedtogether with gypsum in the desulfurizing effluent (refer to referencenumeral 18).

Because reuse or disposal is performed in the form of gypsum mixed withdust as described above, it is not necessary to provide a separate ashtreatment apparatus, thereby simplifying the facility.

Although the above-described embodiments have been described by way ofexample where an oil-fired boiler is used as a boiler, the presentinvention is not limited to this. The present invention is alsoapplicable to boilers that use fuel containing a relatively large sulfurcontent, such as coal-fired boilers.

1. An exhaust-gas treatment apparatus that removes SO₃ and dustcontained in a combustion exhaust gas, comprising: a dry electrostaticprecipitator that removes dust; a dust-dissolving section that suppliesan alkaline solution to dust collected by the dry electrostaticprecipitator to dissolve the dust; and a spray section that sprayseffluent from the dust-dissolving section to an upstream side of the dryelectrostatic precipitator.
 2. The exhaust-gas treatment apparatusaccording to claim 1, wherein a desulfurization apparatus based on alime-gypsum method is provided downstream of the dry electrostaticprecipitator, and desulfurizing effluent from the desulfurizationapparatus is introduced to the dust-dissolving section.
 3. Theexhaust-gas treatment apparatus according to claim 2, furthercomprising: a gypsum separator that separates gypsum from thedesulfurizing effluent from the desulfurization apparatus, wherein theeffluent discharged from the dust-dissolving section is supplied to thegypsum separator.
 4. An exhaust-gas treatment method of removing SO₃ anddust contained in combustion exhaust gas, comprising: supplying analkaline solution to dust collected by a dry electrostatic precipitatorfor removing dust to dissolve the dust; and spraying effluent resultingafter the dust is dissolved to an upstream side of the dry electrostaticprecipitator.